Many different types of light emitting or generating devices utilize optically luminescent materials or ‘phosphors’ to produce a desired light output. Opto-luminescent phosphors may be excited in response to an optical input energy, and in response will re-emit light, although typically the spectral characteristic of the output light is somewhat different than the spectral characteristic of the input light. Phosphors tend to degrade over time due to exposure to heat. However, many applications for phosphor subject the phosphors to heat during device operation.
Consider a solid state lighting device, for a general lighting application, by way of an example. The solid state light sources typically produce light of specific limited spectral characteristics. To change or enhance the spectral characteristic of a solid state light source, for example, to obtain white light of a desired characteristic, one approach currently favored by LED (light emitting diode) manufacturers, utilizes a semiconductor emitter to pump phosphors within the device package (on or in close proximity to the actual semiconductor chip). Another approach uses one or more semiconductor emitters, but the phosphor materials are provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the semiconductor package). At least some opto-luminescent phosphors that produce desirable output light characteristics degrade quickly if heated, particularly if heated above a characteristic temperature limit of the phosphor material.
Hence, phosphor thermal degradation can be an issue of concern in many lighting systems. Thermal degradation of some types of phosphors may occur at temperatures as low as 85° C. Device performance may be degraded by 10-20% or more. The lifecycle of the phosphor may also be adversely affected by temperature.
At least some of the recently developed semiconductor nanophosphors and/or doped semiconductor nanophosphors may have an upper temperature limit somewhere in the range of 60-80° C. The light conversion output of these materials degrades quickly if the phosphor material is heated to or above the limit, particularly if the high temperature lasts for a protracted period.
Maintaining performance of the phosphors therefore creates a need for efficient dissipation of any heat produced during light generation. A current mitigation technique for phosphor thermal degradation is to maintain separation of the phosphor from the heat source and maximize unit area of phosphor to minimize flux density. However, the need for more lumens in an output using the phosphor requires larger phosphor unit area, and any limits placed on the flux density to reduce thermal impact on the phosphor constrains the overall device design.
For equipment utilizing phosphors, there is a continuing need for ever more effective dissipation of heat. Improved heat dissipation may provide a longer operating life for the apparatus or device using the phosphor(s). Improved heat dissipation may allow a device to drive the phosphor harder, to emit more light, for a particular application.
Many thermal strategies have been tried to dissipate heat from and cool active optical elements, including those that have or are combined with phosphors. Many systems or devices use a heat sink to receive and dissipate heat from the hot system component(s) during operation. A heat sink is a component or assembly that transfers generated heat to a lower temperature medium. Although the lower temperature medium may be a liquid, the lower temperature medium often is air.
A larger heat sink with more surface area dissipates more heat to the ambient atmosphere. However, there is often a tension or trade off between the size and effectiveness of the heat sink versus the commercially viable size of the device that must incorporate the sink. For example, if a solid state lamp must conform to the standard form factor of an A-lamp to be a commercially viable product, then that form factor limits the size of the heat sink. To improve thermal performance for some applications, an active cooling element may be used, to dissipate heat from a heat sink or from another thermal element that receives heat from the active system element(s) generating the heat. Examples of active cooling elements include fans, Peltier devices, membronic cooling elements and the like.
Other thermal strategies for equipment have utilized heat pipes or other devices based on principles of a thermal conductivity and phase transition heat transfer mechanism. A heat pipe or the like may be used alone or in combination with a heat sink and/or an active cooling element.
A device such as a heat pipe relies on thermal conductivity and phase transition of a working fluid between evaporation and condensation to transfer heat between two interfaces. Such a device includes a vapor chamber and working fluid within the chamber, typically at a pressure somewhat lower than atmospheric pressure. The working fluid, in its liquid state, contacts the hot interface where the device receives heat input. As the liquid absorbs the heat, it vaporizes. The vapor fills the otherwise empty volume of the chamber. Where the chamber wall is cool enough (the cold interface), the vapor releases heat to the wall of the chamber and condenses back into a liquid. Thermal conductivity at the cold interface allows heat transfer away from the mechanism, e.g. to a heat sink or to ambient air. By gravity or a wicking structure, the liquid form of the fluid flows back to the hot interface. In operation, the working fluid goes through this evaporation, condensation and return flow to form a repeating thermal cycle that effectively transfers the heat from the hot interface to the cold interface. Devices like heat pipes can be more effective than passive elements like heat sinks, and they do not require power and/or mechanical moving parts as do active cooling elements. It is best to get the heat away from the active optical element and any other sensitive components such as a phosphor as fast as possible, and the heat pipe improves heat transfer away from the active optical element, even where transferring the heat to other heat dissipation elements.
Although these prior technologies do address the thermal issues somewhat, there is still room for further improvement, particularly with regard to thermal issues effecting the phosphor or phosphors in light emitting systems.
For example, passive cooling elements, active cooling elements and heat transfer mechanisms that rely on thermal conductivity and phase transition have been implemented outside of the devices that incorporate active optical elements and separate and apart from any phosphor that may be included in the light emitting system. A light processing device may include one or more elements coupled to the actual system element that generates the heat, to transfer heat to the external thermal processing device. Of note, these devices, cooling elements and related thermal mitigation strategies have not been specifically adapted to the cooling of phosphors.
There is an increasing desire for higher, more efficient operation (light output or response to light input) in ever smaller packages. As outlined above, thermal capacity may require control of heat at the phosphor level. Hence, it may be advantageous to improve technologies to more effectively dissipate heat from and/or around phosphor materials.